Just wondering. I use Mr.Malty and/or Yeastcalc for these type questions and both have a 10% viability at 5 months. Where does the 25%/month rule of thumb come from?
The MrMalty calculator assumes that viability drops linearly from 100% to 10% over a period of ~3.5 months, then stays at 10%. Which is obviously a very artificial-looking curve, but probably close enough not to cause any problems until you get to very old samples.
Who originated the 25%/month rule, I don't know, but it does track well with cell counts I've done on stored slurry.